1. Field of the Invention
The present invention relates to an optical coupling device for coupling an output beam from a semiconductor laser to an optical fiber.
2. Description of the Prior Art
In general, in the case that an output beam from a semiconductor laser is coupled to an optical fiber, there is the possibility that a light beam reflected from a discontinuous portion of the optical fiber is incident upon the semiconductor laser. Such an incident light beam adversely affects characteristics such as oscillation, noise and so on of the semiconductor laser.
In order to overcome this problem, the following method can be considered.
(i) An optical fiber is fabricated in such a way that no light beam is reflected from the discontinuous portion of the optical fiber; and
(ii) a non-reciprocal element such as an optical isolator is inserted so that the reflected light beam can be prevented from returning to the semiconductor laser.
However, according to the method (i), no satisfactory result can be obtained unless all the discontinuous portions of the optical fiber are fabricated and there is a defect that it becomes difficult to splice the optical fibers after they are fabricated. In addition, according to the method (ii), there is a defect that an optical isolator is expensive.
Therefore, as shown in FIG. 9, there has been devised a method in which the output beam from a semiconductor laser 1 is converged through a lens 2 and incident upon a single-mode optical fiber 3. The output beam from the single-mode optical fiber 3 is incident upon a multi-mode optical fiber 4. As a result, a reflected beam in the multi-mode optical fiber 4 because of discontinuous portion can hardly be incident upon the single-mode optical fiber 3, so that the reflected beam hardly returns through the lens 2 to the semiconductor laser 1. In this case, however, the single-mode optical fiber 3 is used, so that adjustments with a high degree of accuracy are needed to make the laser beam incident upon the single-mode optical fiber 3. Accordingly, a mechanism for such adjustments becomes complicated and expensive.